FIG. 1 shows a prior-art wireless system. The system includes modulated carrier signals traveling from a transmitter 110 to a receiver 120 following many different (multiple) transmission paths.
Multipath can include a composition of a primary signal plus duplicate or echoed images caused by reflections of signals off objects between the transmitter and receiver. The receiver may receive the primary signal sent by the transmitter, but also receives secondary signals that are reflected off objects located in the signal path. The reflected signals arrive at the receiver later than the primary signal. Due to this misalignment, the multipath signals can cause intersymbol interference or distortion of the received signal.
The actual received signal can include a combination of a primary and several reflected signals. Because the distance traveled by the original signal is shorter than the reflected signals, the signals are received at different times. The time difference between the first received and the last received signal is called the delay spread and can be as great as several micro-seconds. The multiple paths traveled by the modulated carrier signal typically results in fading of the modulated carrier signal. Fading causes the modulated carrier signal to attenuate in amplitude when multiple paths subtractively combine.
In a typical cellular, communications system a base station controls communications within a certain geographic coverage area, termed a cell, and subscribers located within the cell communicate with the base station over wireless channels. The system is conventionally designed to operate using one of various air interface standards that determine the specific protocols, modulation techniques, and multiplexing used for the communication. These existing air interface standards, however, have limitations and trade-offs that have motivated researchers to investigate ways to improve the quality and capacity of cellular communication systems.
In cellular communications systems, the wireless spectrum available for use is a limited resource. It is therefore important to make efficient use of the available bandwidth when developing new cellular systems. Any one of various known schemes may be used to divide the spectrum in frequency and/or time, creating a set of communication channels that may be allocated to subscribers. Frequency division multiple access (FDMA) is a method of dividing the wireless spectrum that associates each communication channel with a different single-frequency carrier. Often the single frequency is further divided in time using time division multiple access (TDMA). In TDMA the frequency carrier is divided into successive time frames, each containing a set of time slots. A single communication channel in an FDMA/TDMA system is thus associated with both a specific carrier frequency and a particular time slot.
Orthogonal frequency division multiplexing (OFDM) is a sophisticated method of FDMA/TDMA. In OFDM, each channel is associated with a time slot and a set of multiple subcarriers multiplexed together, each subcarrier at a different frequency and each modulated by a signal which varies discretely rather than continuously. The set of subcarrier frequencies associated with each channel is chosen from all subcarrier frequencies available to the system. In any multiplexing scheme, channel assignment, or channel allocation is the process of assigning each subscriber to one or more time intervals and/or to one or more frequency carriers or subcarriers. For example, in an OFDM system, two subscribers might be allocated the same time slot, but different frequency subcarriers; or two subscribers might be allocated the same subcarriers, but different time slots; or two subscribers might be allocated both different subcarriers and different time slots. In any case, the channels allocated to the two subscribers are distinct. Thus, the term channel, in the context of channel allocation, is used to refer to a unique time/frequency portion of the spectrum that the system selects and assigns to particular subscribers for their communication needs.
More generally, however, a communications channel includes spatial components in addition to the time and frequency components. In contrast to the time and frequency components of the channel that are freely selected and assigned to subscribers by the system, the spatial components of the channel are not normally under the control of the system, and cannot be assigned or allocated. Thus, channel allocation does not normally include assignment of spatial components of the channel (beamforming systems being an exception). The spatial components of a channel are associated with the path or paths that the wireless signal follows as it propagates through space between the base station and the subscriber. The spatial channel thus depends on the location of the subscriber relative to the base station, and how the environment affects the propagation of the signal (e.g., multi path effects). In addition, the spatial channel will have several spatial subchannels if the base station and/or subscriber has multiple antennas, i.e., in the case of a multiple input, single output (MISO), single input, multiple output (SIMO), or multiple input, multiple output (MIMO) system.
A wireless channel is characterized by a channel response that represents how the transmitted signal is distorted as it propagates through the channel from the transmitter to the receiver. The particular channel response depends on the transmitter, the receiver, and the environment through which the wireless signal passes. For example, the previously described multipath distortions are caused by reflections of the signal from objects in the environment, and Doppler effects distort the signal when there are relative movements between the transmitter, receiver, and objects in the environment.
To compensate for these distortions, communication systems often transmit training sequences over the channel. A training sequence is an information signal that is known a priori by both transmitter and receiver. By comparing the known training sequence with the received training sequence signal that has passed through the channel, the receiver can estimate the channel response. This estimate can then be used by the receiver to compensate for distortions introduced into unknown information signals transmitted over the channel, thereby improving the ability of the receiver to accurately reconstruct the original information signal.
The training sequences are generally transmitted over a subset of the total number of sub-carriers of an OFDM symbol. For example, a single OFDM symbol transmitted within a single time slot may include 1024 sub-carriers. Of the 1024 sub-carriers, 102 of the sub-carriers may include training sequences. Therefore, the channel response at the 102 sub-carriers can be accurately determined. The channel responses of the other 920 sub-carriers can be estimated by interpolating between the nearest training sequence tones.
FIG. 2 shows a prior-art OFDM receiver system. The OFDM receiver system includes an antenna 205 and an OFDM receiver 210 that receive training tones for characterizing the channel response at the frequencies of the training tones. The channel responses of the training tone are convolved or interpolated with a predetermined filter 220 within a convolution unit 230 generating channel responses for the sub-carriers (tones) located between the training tones. A limitation of this receiver system is that the interpolation filter is typically optimized for a generic channel, that may not be optimal for other channel settings. For example, an interpolation filter that is optimal for a low-delay spread channel may provide poor performance for a high-delay spread channel.
Additionally, the interpolation filter is generally sensitive to noise and interference. FIG. 3 shows another prior-art OFDM receiver system. This OFDM receiver system includes an antenna 305 and an OFDM receiver 310 that receive training tones for characterizing the channel response at the frequencies of the training tones. The frequency responses of the training tones for each training time slot are converted to the time domain through an inverse fast Fourier transform (IFFT) unit 320. The time domain response is generally peak detected by a peak detector 330 to eliminate some of the effects of noise. Again, the peak detection occurs for every training time slot. The peak detected response is converted back to the frequency domain through a fast Fourier transform (FFT) unit 340. The characterized channel response is generated by the FFT unit 340.
The primary limitation of the receiver system of FIG. 3 is sensitivity to noise and interference. If a channel is fading, the peak detector 330 can confuse noise and channel multipath. As a result, the time-domain response can be incorrectly estimated, leading to degradation of the estimation of the channel. Another limitation is implementation complexity. The IFFT unit 320 and the FFT unit 340 are generally complex and operationally require large amounts of computation time.
The prior-art training systems are generally sensitive to noise and interference. Fading and multipath can also cause problems. Additionally, multiple carrier systems (such as OFDM) can suffer from time jitter, which limits the performance of prior-art training systems.
It is desirable to have a method and system that provides a robust estimation of a transmission channel that is less sensitive to noise and is inexpensive to implement.